I'm imagining something like a simple 2x12" GSub type ported cabinet but with the 12"s in isobaric push pull configuration, four per cabinet, using a driver like the BMS 12n630, maybe with a slight v-baffle to save some space.

Run 30hz - 120hz.

Opinions? Are their any commercial designs doing something like this? Weight to SPL ratio could be decent.

Isobaric loading will make using smaller box for lower cutoff possible. But you will lose about 3db in sensitivity.

Ahhh ok thought there was a general gain in sensitivity for isobaric, the document above shows this is dependent on frequency with preference to low frequencies.

EDIT: link was referring to manifolds

The on-axis frequency response of the direct-radiating system is
shown in Figure 17 and the response of the manifolded system is
shown in Figure 18. Comparing the two curves, there are several
differences that are readily apparent; below 70 Hz the manifolded
system has substantially more output than the direct-radiating
system; above 100 Hz the manifold has slightly less output; and
above 200 Hz the manifold rolls off abruptly....

....From 70 Hz to 100 Hz the manifolded response is very similar to the direct-radiating case.

Sorry for the confusion caused. Strictly speaking Isobaric and manifold are two different subjects but in practice one may also benefit from manifold loading in an already multiple driver configuration such as an isobaric.

Snowflake, regarding your question of optimum front chamber. The shorter the driver to driver distance is the better the low frequency coupling. For attenuation of unwanted upper harmonics, you can go for a 1/3 wavelength (120 degree phase) of the driver to driver distance. That will be the lowest possible frequency attenuated by destructive interference and attenuation will most likely start to happen at higher frequencies than this.

Snowflake, regarding your question of optimum front chamber. The shorter the driver to driver distance is the better the low frequency coupling. For attenuation of unwanted upper harmonics, you can go for a 1/3 wavelength (120 degree phase) of the driver to driver distance. That will be the lowest possible frequency attenuated by destructive interference and attenuation will most likely start to happen at higher frequencies than this.

the paper above seems to say there is no point putting the drivers closer than the radius of the diaphragm as the reactive component is already infinite at this distance. also the compression ratio at this point would be ~3:1 and likely to cause problems if any less. with an 18" driver and 20cm spacing the attenuation would begin above 570Hz which seems about right. the paper says you have to be careful not to damage mid-band sensitivity but doesn't say any more about it.

Snowflake, regarding your question of optimum front chamber. The shorter the driver to driver distance is the better the low frequency coupling. For attenuation of unwanted upper harmonics, you can go for a 1/3 wavelength (120 degree phase) of the driver to driver distance. That will be the lowest possible frequency attenuated by destructive interference and attenuation will most likely start to happen at higher frequencies than this.

the paper above seems to say there is no point putting the drivers closer than the radius of the diaphragm as the reactive component is already infinite at this distance. also the compression ratio at this point would be ~3:1 and likely to cause problems if any less. with an 18" driver and 20cm spacing the attenuation would begin above 570Hz which seems about right. the paper says you have to be careful not to damage mid-band sensitivity but doesn't say any more about it.

Yes, in the band of a subwoofer or woofer the wavelengths involved allows quite some driver distance.

I did some (to me) quite revealing sims of a hyperbolic horn using 4 18sound 12nd610 in a manifold compression chamber like the ev subs. The system design feature of hornresp suggested a horn of around 160cm and a fairly small throat for a single driver, but simulating 4 drivers decreased optimum horn length to less than 130cm and a throat of around 800sqcm, resulting in a shorter and more open horn than is traditionally considered a loading optimized horn. The 12nd610 is a very light, high BL, low-excursion driver. Other heavier cone/higher power/higher excursion drivers simulated the opposite, requiring a longer horn and higher compression ratio per driver for more drivers.

Unfortunately an "ms sans serif" problem doesn't allow me to enter hornresp to get exact values and take some pics for you at the moment.

My point is that this reactive component of a manifold introducers some very interesting elements of ingeneering including driver selection.

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